What is DNA?
DNA (deoxyribonucleic acid) is the long molecule that stores the instructions of a living thing. It is found mostly in the nucleus, packed into chromosomes. Before reading how it is built, see why it is the genetic material in DNA as genetic material.
The building block: the nucleotide
DNA is a very long chain of small units called nucleotides. Each nucleotide has three parts:
- a phosphate group,
- a sugar called deoxyribose (a five-carbon sugar),
- a nitrogen base: adenine (A), thymine (T), guanine (G) or cytosine (C).
Sugar and phosphate join one after another to make the backbone of a strand. The bases hang inward from the backbone like the rungs of a ladder.
The bases come in two sizes: A and G are purines (two rings, big). T and C are pyrimidines (one ring, small).
Base pairing: A with T, G with C
Two strands face each other and their bases meet in the middle. The pairs are fixed:
- A pairs with T, joined by 2 hydrogen bonds.
- G pairs with C, joined by 3 hydrogen bonds.
Why not A with G? Both are big, so the pair would be too wide and would bulge. A big base with a small base gives the same width (about 2 nm) all along, and the hydrogen bonds line up only for A-T and G-C. A and T are called complementary, and so are G and C.
Hydrogen bonds are weak one by one, but thousands together hold the helix firmly, and they can be opened easily when the cell needs to copy or read the DNA.
The double helix shape and antiparallel strands
- The two strands wind around each other to form a right-handed spiral.
- Width: about 2 nm.
- One full turn: 10 base pairs, which is 3.4 nm long. So the gap between neighbouring base pairs is 0.34 nm.
- Antiparallel: the two strands run in opposite directions. One runs 5' to 3', the other 3' to 5'. The 5' and 3' names come from the carbon atoms in the sugar where the chain starts and ends.
Chargaff's rule and how scientists found the shape
Erwin Chargaff measured the bases in DNA from many species and found: number of A = number of T, and number of G = number of C. So A + G = T + C. The ratio of A + T to G + C is different in different species, which is part of what makes each species unique.
In 1953 James Watson and Francis Crick put together three clues: Chargaff's counts, the X-ray pictures of Rosalind Franklin and Maurice Wilkins, and the sizes of the bases. They built a model of a double helix with A-T and G-C pairs. It explained at once how DNA can be copied: each strand holds the information needed to make the other.
Building a model of DNA
To build a model, follow these steps:
- Make one strand: write a sequence of bases, for example A T G C.
- Find each partner by the rule (A-T, G-C): T A C G.
- Join the two strands with rungs and keep them antiparallel.
- Give the ladder a right-handed twist, one turn for every 10 pairs.
Try this in the 3D on the last step.
Try it: make a candy DNA model
At home: take 4 colours of soft candy or clay balls: A = red, T = yellow, G = green, C = blue. Use two long sticks (or straws) as the backbone. Make a strand of 8 bases on one stick, then fill the other stick with the partners. Check each pair: red with yellow, green with blue. Now twist the two sticks around each other, one turn after 10 pairs.
In the 3D: on the last step choose each partner base. If you pick a wrong one the model tells you. When all 12 are right, the ladder twists by itself.
Key formulas and definitions
- Base pairs: A = T (2 hydrogen bonds), G ≡ C (3 hydrogen bonds)
- Chargaff: A = T, G = C, so A + G = T + C
- One turn = 10 base pairs = 3.4 nm; one base pair = 0.34 nm; width = 2 nm
- Length of DNA (nm) = number of base pairs × 0.34
- Hydrogen bonds = 2 × (A-T pairs) + 3 × (G-C pairs)
Worked examples
1. One strand reads 5'-ATGCCA-3'. Write the other strand.
Pair each base: A-T, T-A, G-C, C-G, C-G, A-T. The partner strand is TACGGT (reading it 3' to 5' under the first, as the strands run opposite ways). Written 5' to 3' it is TGGCAT.
2. A DNA sample has 30% adenine. Find the percentages of T, G and C.
A = T, so T = 30%. A + T = 60%, leaving 40% for G + C. G = C, so G = 20% and C = 20%.
3. A piece of DNA has 8 A-T pairs and 6 G-C pairs. How many hydrogen bonds hold it? And how long is it?
Bonds: 8 × 2 = 16 and 6 × 3 = 18, total 34. Length: 14 base pairs × 0.34 nm = 4.76 nm.
Common mistakes
- Pairing A with G or C with T. Only A-T and G-C fit.
- Saying A-T has 3 hydrogen bonds. A-T has 2, G-C has 3.
- Thinking the percentages of A and G are equal. A = T and G = C; A and G are usually different.
- Forgetting the strands are antiparallel: one runs 5' to 3', the other 3' to 5'.